Silver halide photography usually involves the exposure of silver halide with light in order to form a latent image that is developed during photographic processing to form a visible image. Silver halide is intrinsically sensitive only to light in the blue region of the spectrum. Thus, when silver halide is to be exposed to other wavelengths of radiation, such as green or red light in a multicolor element or infrared radiation in an infrared-sensitive element, a spectral sensitizing dye is required. Sensitizing dyes are chromophoric compounds (usually cyanine dye compounds) that are adsorbed to the silver halide. They absorb light or radiation of a particular wavelength and transfer the energy to the silver halide to form the latent image, thus effectively rendering the silver halide sensitive to radiation of a wavelength other than the blue intrinsic sensitivity.
In many situations, it is also desirable to sensitize silver halide to blue light. This can be because the particular silver halide emulsion has low intrinsic sensitivity, because it is desired to alter the spectral response of the silver halide in the blue portion of the spectrum, or simply because greater blue sensitivity in addition to the intrinsic sensitivity of the silver halide is desired.
Cyanine dyes have often been proposed as blue sensitizers, but due to their tendency to cause post-processing retained dye stain, merocyanines have often been used instead. U.S. Pat. No. 2,153,169 describes one such merocyanine dye having the formula: ##STR2## Such dyes, however, often do not provide silver halide with as high a sensitivity to blue light as may be desired, and they still exhibit some retained dye stain. It is thus an object of the present invention to provide a low-staining effective blue sensitizing dye for silver halide photographic materials.